Optical couplers are well known which are photon-coupled devices in which an electrical signal is converted into light that is projected through an insulating interface and reconverted to an electrical signal. Various forms of optical couplers are well known, such as a light emitting diode and photo diode combination, a light emitting diode and photo-transistor combination, a light emitting diode and photo-Darlington combination, a light emitting diode and LDR combination, a neon light and LDR combination, a lamp and LDR combination and a light emitting and diode photo diode driving a transistor switch combination. Optical couplers have a switching characteristic having a fast turn on and a slower turn off. As a result, optical couplers require less time to turn on than to turn off. Diverse switching applications exist in which the electrical isolation provided by an optical coupler is desirable to prevent undesired electrical coupling between a switching signal source and a load switch whose conductivity is being controlled by the switching signal source. An example of such an application is in the drive circuit for a variable reluctance multiple phase electrical motor. Many switching applications in which an optical coupler may be utilized require both a fast turn on and a fast turn off characteristic to optimize performance. Undesirably, a fast turn off characteristic may not be obtained with a single optical coupler controlling both the turning on and the turning off of a load switch (a switch controlling current flow through an electrical load).
FIG. 1 illustrates an example of a prior art drive circuit for a load switch using an optical coupler. A switching signal source 10 provides switching pulses which control the conductivity of a Darlington amplifier 12 comprised of a first bipolar transistor 14 and a second bipolar transistor 16. The switching pulses produced by the switching signal source 10 vary between a first low level and a second high level. The Darlington amplifier 12 is conductive when the switching signal produced by the switching signal source 10 is at the second level. The Darlington amplifier 12 controls the flow of current through an electrical load 18 which may be diverse in nature such as the windings of a single phase of a variable reluctance motor. The switching signal from the switching signal source 10 is electrically coupled to optical coupler 20 which may be any known optical coupler configuration. As described above, the optical coupler provides electrical isolation between the switching signal source and the Darlington amplifier 12. When the switching signal reaches the high level, the optical coupler turns on to produce a high level output signal The turn on characteristic of the optical coupler is relatively fast when compared to the turn off characteristic which occurs when the switching pulse falls from the second level to the first level. The output of the optical coupler is coupled to a first channel 24 which is comprised of amplifier 26 and switch 28. The switch 28 is conductive when the switching signal is at the second level which causes the Darlington amplifier 12 to turn on with a relatively fast turn on characteristic. The output of the optical coupler 20 is coupled to a second channel 30 comprised of inverting amplifier 32 and bipolar transistor 34. The inverting amplifier 32 produces a high level output signal when the switching signal pulse is at the first low level which causes the Darlington amplifier to turn off. However, as a consequence of the relatively slow turn off characteristic of the optical coupler 20, the optical coupler 20 does not rapidly change state in response to the switching signal source falling from the higher second level to the first level which slows down the turn off characteristic of the Darlington amplifier 12.